A wireless transmit/receive unit (WTRU) in a universal terrestrial radio access network (UTRAN) may be in either an idle state or a connected state. While the WTRU is in a connected state, based on WTRU mobility and activity, the UTRAN may direct the WTRU to transition between Cell_PCH, URA_PCH, Cell_FACH, and Cell_DCH states. User plane communication between the WTRU and the UTRAN is only possible while the WTRU has a radio resource control (RRC) connection to the UTRAN.
The Cell_DCH state is categorized by dedicated channels in both uplink and downlink. On the WTRU side, this corresponds to continuous transmission and reception and may be demanding on user power requirements.
As defined in Release 6 of the Third Generation Partnership Project (3GPP) specifications, the Cell_FACH state does not use dedicated channels and thus allows better power consumption at the expense of a lower uplink and downlink throughput. While in the Cell_FACH state, uplink communication is achieved through a random access channel (RACH) while downlink communication is through a shared transport channel, (e.g., a forward access channel (FACH)), mapped to a secondary common control physical channel (S-CCPCH). The Cell_FACH state is suited for signalling traffic, (e.g., transmission of cell update and UTRAN registration area (URA) update messages), and for applications requiring very low uplink throughput.
While in the Cell_FACH state, the WTRU may perform signal measurements and/or traffic volume measurements (TVM) as specified in the measurement control information. The signal measurement is used by the WTRU for cell reselection. The TVM is reported to the UTRAN within a measurement report based on criteria specified on the measurement control information. The measurement report is sent via the RACH.
The RACH is based on a slotted-Aloha mechanism with an acquisition indication. Before sending an RACH message, a WTRU attempts to acquire the channel by sending a short preamble (made up of a randomly selected signature sequence) in a randomly selected access slot. After transmitting the RACH preamble, the WTRU waits for an acquisition indication from the UTRAN. If no acquisition indication is received, the WTRU ramps up the transmit power for the RACH preamble and retransmits the RACH preamble, (i.e., sends a randomly selected signature sequence in a selected access slot). If an acquisition indication is received, the WTRU has effectively acquired a channel, and may transmit an RACH message. The initial transmit power for the RACH preamble is set based on an open loop power control technique, and the ramp-up mechanism is used to further fine-tune the WTRU transmit power.
It has been proposed to use high speed downlink packet access (HSDPA) in a Cell_FACH state. HSDPA is a feature that was introduced in Release 5 of the third generation partnership project (3GPP) specifications. HSDPA operates in a Cell_DCH state. HSDPA makes better use of the downlink shared capacity by using three key concepts: adaptive modulation and coding (AMC), retransmissions using a hybrid automatic repeat request (HARQ) scheme, and Node-B scheduling.
Every two (2) milliseconds, the Node-B schedules transmissions on the high speed downlink shared channel (HS-DSCH) based on information the Node-B collects from the WTRUs and the status of the downlink buffers. In addition, the Node-B tailors the transmission bit rates to the specific WTRUs by adapting the MCS, transport block size, etc. The Node-B may transmit at a higher data rate to those WTRUs which perceive favorable channel conditions, and a lower data rate to those WTRUs that perceive unfavorable channel conditions, (e.g., at the cell edge).
For the HSDPA operations, the Node-B needs channel quality indication (CQI) and positive-acknowledgement (ACK)/negative-acknowledgement (NACK) feedback from the WTRUs. The CQI is an index into a table which provides the maximum MCS that the WTRU may support. The CQI is sent periodically with periodicity determined by the UTRAN. The ACK/NACK feedback is for the HARQ process. The ACK/NACK information is only provided in response to a packet being received on the downlink.
In Release 6 of the 3GPP specifications, the CQI and ACK/NACK information are transmitted via a high speed dedicated physical control channel (HS-DPCCH). Each WTRU is assigned a separate HS-DPCCH and as a result a WTRU may easily provide the feedback information. Moreover, the HS-DPCCH is power controlled using an offset to the uplink dedicated physical control channel (DPCCH), for which close loop power control is performed. The information on the HS-DPCCH is heavily coded to aid in detection. As more and more WTRUs use HSDPA, the number of feedback control channels increases. Even if these are power controlled, the feedback information may cause an uplink noise rise, reducing the capacity available for other uplink transmissions.
If HSDPA is to be used in a Cell_FACH state, the main problem is a lack of dedicated uplink channel to transmit the CQI and ACK/NACK information. Without the CQI and ACK/NACK information, the advantages of HSDPA are significantly reduced. 3GPP Release 6 specifications do not provide support for optimal MCS selection and scheduling for the HS-DSCH in a Cell_FACH state.
Therefore, it would be desirable to provide a method and apparatus for providing CQI information via a shared channel in a Cell_FACH state.